Long a hangout for college students, the social-networking giant has morphed into a virtual parlor for the middle-aged, according to a new study.

People 35 to 54 are now the biggest age group on the Web site, accounting for 28.2 percent of all U.S. users as of July, according to iStrategyLabs, an online marketing firm. Following close behind are 24- to 34-year-olds, who represent 25.2 percent of users.

The findings reflect a major evolution at Facebook, which until January was dominated by young adults, some of whom famously festoon their profiles with photos of beer bashes and belly buttons. Although their numbers continue to grow on the service, younger users have been overtaken by their seniors, who are joining at a faster rate.

The 18-to-24 age group accounted for 25.1 percent of users, down from 40.8 percent in January, the study found. Those 17 and under made up 9.8 percent of the Web site's users, down from 13.5 percent.

Networking cited

Peter Corbett, chief executive of iStrategyLabs in Washington, D.C., attributed the influx of older people to job networking amid the gloomy economy. He also pointed to the 2008 presidential campaign, during which the candidates and media outlets like CNN invited people to connect.

Corbett said the influx of middle-age users raises the question of whether Facebook can retain its younger audience. How cool can a Web site be after Mom and Dad join?

"Does this younger audience now leave Facebook and try to find their own place where they can be themselves?" Corbett asked.

Facebook was co-founded in 2004 by Mark Zuckerberg, a Harvard University student, as a place for his peers to keep in touch with one another. Even after opening its doors to everyone, the Web site retained a youthful image while luring an increasing number of middle-age users.

But it took until now for the demographic balance of power to formally shift. People 25 and older make up the majority of users (61.5 percent), placing the site squarely in the Web's mainstream.

The study was based on numbers Facebook publicly supplies to its advertisers, many of whom target their ads based on user demographics. The data isn't exact, although it does give a general view of who's using Facebook.

Since January, the number of new users between ages 35 to 54 grew 190 percent, to 20.2 million, according to the study. New users 55 and older grew 590 percent, to 5.9 million.

Joel W. Gonzales, director of admissions at the UCSF pharmacy school, said he joined Facebook last year, after hearing his colleagues and some of the graduate students rave about it. The 40-year-old said he has used the Web site to get in touch with people from high school, post photos of his travels and talk about everything from politics to celebrities.

"I've been sucked into it," Gonzales said.

Tapering off

Among younger users, Facebook's growth is tapering off. The ranks of 18 to 24 year olds, for instance, increased only 4.8 percent since January, according to the study.

At least some of the sluggishness can be attributed to the fact that so many young adults have already joined Facebook. Some may be defecting to rival sites like Twitter, the San Francisco microblogging service, but their numbers are believed to be limited so far.

Richard Park, a 21-year-old who recently graduated from UC Berkeley, said he was unaware that Facebook's demographics were increasingly skewing to the middle-aged. To him, the average user still seemed to be in college.

But Park acknowledges that he has been adding a few older friends to his network, figuring it could only help his chances when he starts looking for a job. He feels no pressure to quit the site or hide college party photos simply because of age creep.

"I really like using Facebook," Park said. "They've got a good system, and I see no reason to change."

Attractive for ads

If anything, the influx of older users makes Facebook a more attractive place for advertisers, said Corbett, the study's author. There's a lot of hype about the attractive 18-to-24 demographic, he said, but its people who are older who have more money to spend.

"Do you want a massive population of wealthy Baby Boomers who have disposable income or a bunch of poor college kids?" Corbett said. "The audience that is growing now on Facebook is a really valuable one to have."